Known devices based on the above principle possess three electrodes and three connection wires to the control system, with the control system often being protected from the liquid by a liquid-proof wall through which the wires pass. Often only two wires pass through the wall, if the wall itself is made of metal and serves as a reference potential common electrode, but two wires nevertheless remain necessary for connecting to the electrodes at high level and at low level. In addition to the problem of passing through the wall, there is also the difficulty of positioning the electrodes and of adjusting their heights in order to change one of the levels relative to the other.